Slot antenna for mm-wave signals

ABSTRACT

The present invention relates to an antenna ( 1 ) for radiating/or receiving mm-wave signals, comprising a substrate ( 2 ), a planar conducting layer ( 3 ) formed on said substrate ( 2 ), and a radiation element ( 4 ) being formed as a slot in said planar conducting layer ( 3 ), said slot comprising a middle part ( 4   a ) and two outer parts ( 4   b ) being connected by said middle part ( 4   a ) and extending away from said middle part ( 4   a ), said antenna further comprising a feeding structure ( 5 ) adapting to feed signals to said middle part ( 4   a ) of said slot. The antenna provides a low-cost structure with a high gain.

The present invention relates to a slot antenna for radiating and/or receiving mm-wave signals. Specifically, the present invention relates to a slot antenna which is adapted to transmit and/or receive electromagnetic signals in a wireless communication system operating in a high-frequency range, such as the GHz frequency range or the mm wavelength range and is suited for high data rate communication.

The object of the invention is hereby to propose such a slot antenna for radiating and/or receiving mm-wave signals which has a simple structure and can therefore be produced at low-cost while still being adapted to be used in a high frequency bandwidth and for high data rate applications.

The above object is achieved by an antenna for radiating and/or receiving mm-wave signals as defined in the enclosed independent claim 1. The antenna according to the present invention comprises a substrate, a planar contacting layer formed on said substrate and a radiation element being formed as a slot in said planar contacting layer, said slot comprising a middle part and two outer parts being connected by said middle part and extending away from said middle part, said antenna further comprising a feeding structure adapted to feed signals to the middle part of said slot.

The antenna of the present invention therefore has a simple structure and can be manufactured at low-cost while still providing a very good performance for high data rate applications in high frequency bandwidth.

It is to be understood that the antenna of the present invention could be used as a pure receiving antenna or a pure radiating/transmitting antenna, or could be used in applications in which electromagnetic signals are radiated from as well as received by the antenna.

The antenna of the present invention is particularly suitable for high frequency bandwidth applications, i.e. applications in the GHz frequency range, such as a frequency range between 20 and 120 GHz. These frequency ranges typically enable high data rate applications since they provide a large frequency bandwidth availability. It would, however, also be possible to use the antenna of the present invention in different frequency ranges and bandwidths depending on the wanted application.

Hereby, by varying the measures of the antenna of the present invention, such as the width and the length and the proportions of the different elements of the present antenna, a specific adaptation to the respectively required frequency range and bandwidth can be achieved. Also, the simple structure and low-cost solution of the antenna of the present invention makes the antenna specifically useful for consumer electronic applications. However, the antenna of the present invention can also be used in other applications if wanted and/or necessary.

Advantageous sub features of the present invention are defined in the dependent claims.

Advantageously, two outer parts of the slot are parallel to each other. Further advantageously, the middle part and the two outer parts form a U together. In other words, the slot has a U-shape. Such a shape is advantageous as it leads to the radiation of electromagnetic signals with linear polarization. Signals with linear polarization are advantageous for indoor applications, specifically for indoor with line of sight and also for non line of sight signals. Such an antenna shape, however, may also be advantageous in selected outdoor applications. The U-shape of the slot leads to a quite large frequency bandwidth around the operation frequency of about 10 percent. For example, in case that the operation frequency is around 60 GHz the achieved frequency bandwidth is around 6 GHz with such a shape. Further advantageously, the width of each of the two outer parts of the slot increases in the direction away from the middle part. By such a tapering of the two outer parts, the antenna impedance can be reduced and matched to the impedance of the feeding structure, which is typically 50 Ohm.

Alternatively, the width of each of the two outer parts of the slot can remain constant, i.e. untapered.

Further advantageously, both outer parts of the slot have the same length and width. In other words, the two outer parts could be mirror-symmetric in relation to a symmetry axis extending between the two outer parts and perpendicular to the middle part of the slot. Further advantageously, the width of each of the two outer parts of the slot is more than two times of the width of the middle part. Further advantageously, the distance between the two outer parts, i.e. the length of the middle part, is larger than the width of each of the two outer parts. Further advantageously, each of the two outer parts is longer than wide.

Further advantageously, the feeding structure is a microstrip feeding line arranged on a side of said substrate opposite to the planar conducting layer. Hereby, the decoupling of the feeding structure from the radiation element has the advantage of suppressing side lobes in the antenna characteristics as compared to structures in which the feeding structure is placed in the same layer as the radiation element. Thus, in the antenna of the present invention, only the shape of the radiation slots determines the antenna radiation pattern, since the side lobe radiation is greatly reduced and therefore the axial ratio of the radiation pattern is greatly decreased, so that the antenna of the present invention is particularly advantageous to be used in an array antenna in which a high gain can be realized and in which the radiation beam can be steered.

Further advantageously, the planar conducting layer and/or the feeding structure are printed elements. By printing the planar conducting layer, for example a copper layer, onto a single layer substrate, the slot can be simply etched with simple etching technology, so that a low-cost structure is achieved. If additionally a simple 50 Ohm microstrip feeding line is printed onto the opposite side of the substrate, i.e. onto the other side opposite to the planar conducting layer, a simple and low-cost feeding structure is achieved.

Further advantageously, the antenna of the present invention has a reflector plane arranged in a predefined distance from the side of the substrate opposite to the planar conducting layer. Such a reflector plane arranged below the antenna is advantageous to avoid backside radiation and is helpful to direct the radiation pattern to the side of the substrate on which the planar conducting layer with the slot is located, therefore increasing the antenna gain in one direction. Between the reflector plane and the substrate, a low dielectic material or air can be provided.

Advantageously, the length and width dimensions of the planar conducting layer are in the range of half of the wavelength of the operation frequency. These dimensions make the antenna of the present invention quite suitable for applications in the mm-wave frequency range.

The present invention is further directed to an antenna array comprising a plurality of antennas according to the present invention. Hereby, the plurality of antennas advantageously have a common substrate and the radiation direction can be changed. For example, the antenna array may comprise beam steering elements adapted to change the radiation direction of each of the antennas. Advantageously, the beam steering elements hereby comprises phase shifters adapted to shift the signal face for each antenna.

Particularly, the arrangement of the feeding structure on the substrate side opposite to the side on which the planar conductive layer is located and therefore decoupling the feeding network from the radiation structure suppresses the side lobes in the radiation patterns so that an antenna array with a very high gain can be achieved. Further, a very reliable beam steering with a high accuracy can be provided due to the fact that—if at all—only very small side lobes are present.

The present invention will be further explained on the basis of the following description of advantageous embodiments relating to the enclosed drawings, in which

FIG. 1 shows a perspective view of an embodiment of an antenna according to the present invention,

FIG. 2 shows a perspective view of the planar conductive layer and the feeding structure of the embodiment of FIG. 1,

FIG. 3 shows a top-view of the embodiment of FIGS. 1 and 2,

FIG. 4 shows an antenna gain versus frequency diagram of the antenna of the previous figures,

FIG. 5 shows a polar plot of the antenna of the previous figures in the E-plane,

FIG. 6 shows a polar plot of the antenna of the previous figures in the H-plane,

FIG. 7 shows a voltage standing wave ratio versus frequency of the antenna of the previous figures,

FIG. 8 shows a perspective view of an embodiment of a beam steering antenna array according to the present invention,

FIG. 9 shows a functional bloc diagram of the beam steering antenna array of FIG. 8,

FIG. 10 shows a diagram of an antenna gain versus frequency of the embodiment of FIGS. 8 and 9, and

FIG. 11 shows a polar plot of the antenna array of FIGS. 8 and 9 with a steered beam.

FIG. 1 shows a perspective view of an embodiment of an antenna 1 for radiating and/or receiving mm-wave signals of the present invention. The antenna has a high gain directional radiation pattern within predetermined frequency bandwidth of operation and is connectable for example to analogue front-end circuitry of a wireless RF transceiver. The antenna is designed to advantageously operate in the GHz frequency range, more specifically in the 20 to 120 GHz frequency range, even more specifically in the 50 to 70 GHz frequency range, and most specifically in the 59 to 65 GHz frequency range. However, the antenna operation is not limited to these frequency ranges, but can be adopted to operate in different frequency ranges by a corresponding downsizing or upsizing of the antenna measures and ratios.

The antenna 1 comprises a substrate 2 which can be formed from any suitable material, such as a dielectric material or the like, and may be formed as a single layer. A planar conducting layer 3 is formed on the substrate 2, for example, by forming a copper layer on the upper side of the substrate 2, for example by a printing technique. In the planar conducting layer 3, a radiation element 4 is formed, which has the shape of a slot. The slot is for example formed by etching technology.

On the side of the substrate 2 opposite to the conducting layer 3, a feeding structure 5 is provided, by which electromagnetic signals are supplied to the radiation element 4 in order to be transmitted or by which electromagnetic signals received by the radiation element 4 are supplied to processing circuitry connected to the feeding structure. Further, in a predetermined distance from the side of the substrate 2 on which the feeding structure 5 is provided, a reflector plane 6, formed by a conducting, for example metal, plane is located. The reflector plane operates as an electromagnetic wave screen to reflect electromagnetic waves transmitted and/or received by the radiation element 4 to cancel or suppress radiation on the backside of the substrate 2 and to increase the antenna gain in the main direction of the antenna, which is the direction perpendicular to the plane of the conducting layer 3 pointing away from the substrate 2. There might be applications, however, in which the antenna of the present invention can be implemented without such a reflector plane 6.

The feeding structure 5 can be any kind of suitable feeding structure, but is advantageously embodied as a microstrip feeding line which is applied to the backside of the substrate 2 by printing technology. Hereby, the microstrip feeding line advantageously has a 50 Ohm impedance.

The operation principle of the antenna 1 of the present invention is as follows. An exciting electromagnetic wave is guided to the radiation element 4 through the feeding structure 5. In the radiation element 4, i.e. the slot, the magnetic field component of the exciting electromagnetic wave excites an electric field within the slot. Hereby, in order to achieve a large frequency bandwidth at the operation frequency, for example a frequency bandwidth of 10 percent of the operation frequency, the radiation element 4 according to the present invention comprises a middle part 4 a and two outer parts 4 b which are connected by said middle part 4 a and extend away from said middle part 4 a, so that a slot antenna is formed. The specific shape of the radiation element is shown in more detail in the perspective view of the planar conductive layer 3 and the feeding structure 5 of FIG. 2 and the top view of the antenna 1 in FIG. 3.

In the shown embodiment of the antenna 1, the slot of the radiation element 4 generally has a U-shape, in which the two arms of the U are formed by the mentioned outer parts 4 b and the base connecting the two outer parts 4 b is formed by a middle part 4 a. The two outer parts 4 b generally extend parallel to each other and perpendicular to the middle part 4 a. The shown U-shape of the slot leads to the frequency bandwidth of approximately 10 percent of the operation frequency, for example a frequency bandwidth of 6 GHz and an operation frequency around 60 GHz. In the shown embodiment, the transition between the middle part 4 a and the two outer parts or arms 4 b is rounded. However, in different applications, the transition between the middle part 4 a and the two outer parts 4 b could be rectangular with corners.

As indicated in FIG. 2, the shape of the planar conductive layer and thus the substrate 2 is generally rectangular with equally long sides rl1 and rl2 presenting a quadratic shape. However, different shapes could be applied in which rl1 is smaller or larger than rl2.

FIG. 3 which is a top-view of the antenna 2 also shows the feeding structure 5 on the backside of the substrate 2 unlashed lines in order to show the arrangement of the feeding structure 5 in relation to the radiation element 4. Specifically, the feeding structure 5, in the shown embodiment a printed microstrip line, feeds or leads signals away from the middle part 4 a of the radiation element 4. Hereby, the feeding structure is located on the backside of the substrate 2 opposite to the planar conductive layer 3 and the slot 4, so that the feeding structure and the radiation element are decoupled in order to suppress side lobes of the radiation characteristic. The feeding structure 5 hereby feeds signals to the middle part 4 a of the radiation element 4 from a direction which is opposite to the direction in which the two outer parts 4 b of the radiation element 4 extend. In the two dimensional projection visualized in FIG. 3, it can be seen that the feeding structure 5 overlaps with the middle part 4 a of the radiation element 4 in order to ensure a good coupling across the substrate 2.

The planar conductive layer 3 and thus the substrate 2 have two symmetry axis A and B which split the conductive layer 3 in half in the length as well as in the width direction. Hereby, the feeding structure 5 extends along and symmetrically to the symmetry-axis A and the slot of the radiation element 4 is arranged mirror symmetrically to axis A. In other words, the two outer parts 4 b of the radiation element 4 extends generally parallel to the axis A and are mirror symmetric with respect to it. The base line of the middle part 4 a of the radiation element 4 is arranged on the symmetry axis B. In other words, the distance between the base line of the middle part 4 a is half of the length of the conducting layer 3 in this direction.

Generally, it is advantageous, if the two outer parts 4 b are tapered, i.e. if the width of the two outer parts 4 b increases away from the middle part 4 a. Hereby, the imaginary part of the complex impedance of the radiation element can be decreased so that the over all impedance of the antenna 1 is decreased and can be matched to the impedance of the feeding structure of for example 50 Ohm.

Further, in case that the two outer parts 4 b are tapered, the width w1 of the two outer parts at their ends is larger than the width w2 of the middle part 4 a. Advantageously, the width w1 of the ends of the two out parts 4 b is more than two times larger than the width w2 of the middle part 4 a. Further, the length 13 of the middle part 4 a is larger than the width w1 of the ends of the two outer parts 4 b. In other words, the distance between the two outer parts 4 b is larger than the respective width w1. Further, the over all width w3 of the radiation element 4 is larger than its length 12, whereby each of the two outer parts 4 b has a length 12 which is longer than its width w1. The shown shape and dimensions of the planar conducting layer 3 and the radiation element 4 are particularly suitable for radiating and receiving signals in the 50 to 70 GHz frequency range. FIG. 4 visualizes an antenna gain versus frequency plot of the embodiment of the antenna 1 of the present invention shown in FIGS. 1, 2 and 3. It can be seen, that an antenna gain above 8 dBi can be achieved between 55 and 65 GHz with a single antenna 1 as explained. FIG. 4 shows a polar plot of the antenna 1 in the E-plane and FIG. 5 shows a polar plot of the antenna 1 in the H-plane. It can be seen that the antenna 1 of the embodiment shown in FIGS. 1, 2 and 3 shows a 3 dB HPBW (Half power beam width at 3 dB lower than the maximum gain) of more than 80 degrees in the E-plane and 62 degrees in the H-plane. FIG. 6 shows a VSWR (Voltage standing wave ratio) representing the matching of the antenna 1 which is less than 2 in a frequency bandwidth between 59 and 65 GHz, so that a bandwidth of approximately 10 percent of the operation frequency (approximately 62 GHz) is therefore achieved.

FIG. 8 shows a perspective view of an embodiment of an antenna array 10 in which the antenna 1 of the present invention can be implemented. The antenna array 10 of FIG. 8 shows the implementation of four antennas 1 in a quadratic structure on a common substrate 7. In other words, the common substrate 7, which is for example a single layer substrate similar to substrate 2, has four planar conductive layers printed on its top-side, each of the planar conductive layers comprising a radiation element 4. The feeding structure of the antenna array 10 corresponds to the feeding structure 5 shown and explained in relation to the antenna 1 of FIGS. 1, 2 and 3. Similarly, the antenna array 10 also may comprise a reflector plane 8, being for example a metallic layer being located in a predetermined distance from the substrate 7. However, the reflector plane 8 can also be omitted depending on the application. All elements, functionalities and characteristics explained in relation to the antenna 1 of FIGS. 1, 2 and 3 also apply to the antenna array 10 comprising several antennas 1 as shown in FIG. 8. Instead of four antennas 1, a higher or lower number of antennas 1 can be provided in the antenna array 10 of the present invention. Hereby, the antenna array 10 may have a quadratic structure with identical length rl3 and width rl4 of e.g. 4.5 mm. However, the antenna array 10 can also have different length and width.

FIG. 9 shows a functional bloc diagram of the antenna array 10 with four antennas 1. Each of the antennas 1 has an allocated phase-shift element 9, for example a phase-shifter bank, by means of which the phase of the respective antenna can be changed in order to change the over all radiation pattern of the antenna array 10. Hereby, changing the phase input of each antenna 1 and then steering the individual radiation patterns of each antenna 1, the over all radiation pattern of the antenna array 10 can be steered within a specific angular range around the main lobe direction, which is the direction perpendicular to the plane of the planar conductive layers of antennas 1 away from the substrate 7. FIG. 9 shows a suggestion for a specific implementation and circuitry in order to realize the beam steering possibility. Each phase shifter 9 is connected to its respective antenna via the RF switch 11. Further, each phase shifter 9 is connected to a respective power divider 13 by means of another RF switch 12. The two power dividers 13 are connected to a main power divider 14. The power dividers 14 and 13 are used to divide (in case of using the antenna 10 as transmit antenna array) or to sum (in case of using the antenna array 10 as receive antenna array) an equal signal strength to the four antennas 1 (in case of transmitting) or to an analogue RF front-end (in case of receiving). Additionally, a feeding structure (not shown) such as microstrip lines is used as feeding lines for each antenna 1, identical to the feeding structure 5 explained in relation to antenna 1 of FIGS. 1, 2 and 3.

The phase shifters 9 are used to shift the signal phase at each antenna 1 in order to obtain the desired beam steering pattern direction. Any kind of broad bandwidth microstrip phase shifter can be used and implemented with the antenna array 10 in order to steer the beam pattern. FIG. 10 shows an antenna array gain versus frequency plot for the antenna array of FIG. 8. It can be seen that the antenna array 10 provides a gain of more than 12 dBi in the frequency range between 55 and 65 GHz. FIG. 11 shows a polar plot of the antenna for a steering angle of 30 degrees.

The shape instructor of antenna 1 of the present invention is therefore particularly useful and advantageous for implementation in antenna arrays, such as antenna array 10, with beam steering due to the simple and low-cost structure and the high gain in GHz frequency range. 

1. Antenna (1) for radiating and/or receiving mm-wave signals, comprising a substrate (2), a planar conducting layer (3) formed on said substrate (2), and a radiation element (4) being formed as a slot in said planar conducting layer (3), said slot comprising a middle part (4 a) and two outer parts (4 b) being connected by said middle part (4 a) and extending away from said middle part (4 a), said antenna (1) further comprising a feeding structure (5) adapted to feed signals to said middle part (4 a) of said slot.
 2. Antenna (1) according to claim 1, wherein said two outer parts (4 b) are parallel to each other.
 3. Antenna (1) according to claim 1, wherein said middle part (4 a) and said two outer parts (4 b) have a U-shape.
 4. Antenna (1) according to claim 1, wherein the width (w1) of each of the two outer parts (4 b) increases in a direction away from said middle part (4 a).
 5. Antenna (1) according to claim 1, wherein the width of each of the two outer parts (4 b) is constant.
 6. Antenna (1) according to claim 1, wherein both outer parts (4 b) have the same length (l2) and width (w1).
 7. Antenna (1) according to claim 1, wherein the width (w1) of each of the two outer parts is more than two times the width (w2) of said middle part (4 a).
 8. Antenna (1) according to claims 1, wherein the distance (13) between the two outer parts (4 a) is larger than the width (w1) of each of the two outer parts (4 b).
 9. Antenna (1) according to claim 1, wherein each of the two outer parts (4 b) is longer than wide.
 10. Antenna (1) according to claim 1, wherein said feeding structure (5) is a microstrip feeding line arranged on a side of said substrate (2) opposite to the planar conducting layer (3).
 11. Antenna (1) according to claim 1, wherein said planar conducting layer (3) and said feeding structure (5) are printed elements.
 12. Antenna (1) according to claim 1, wherein said slot is adapted to radiate signals with linear polarization.
 13. Antenna (1) according to claim 1, having a reflector plane (6) arranged in a predefined distance from a side of said substrate (2) opposite to the planar conducting layer (3).
 14. Antenna (1) according to claims 1, wherein the length and width dimensions of the planar conducting layer (9) are in the range of half of the wavelength.
 15. Antenna array (10) comprising a plurality of antennas (1) according to claim 1 having a common substrate (7), said antenna array (10) being steerable.
 16. Antenna array (10) according to claim 15, comprising beam steering elements (9) adapted to change the radiation direction of the each of the antennas (1).
 17. Antenna array (10) according to claim 15, wherein the beam steering elements (9) comprise phase shifters adapted to shift the signal phase for each antenna (1). 